In ink jet printers, ink jet print cartridges or pens are reciprocated on a carriage to print swaths on an advancing media sheet. Pens typically include an ink chamber partially filled with ink, with a print head having an array of nozzles for expelling ink droplets in a controlled pattern. For color printing, existing pens typically use three different color inks to provide a range of colors when printed in different proportions. In addition, many printers use separate black ink cartridges.
During normal printer operations, it is necessary to wipe the surface of the print head. This may occur before printing begins, at intervals during printing, and after printing concludes. Wiping serves to remove any debris, and to "prime" any dry nozzles by the capillary or wicking effect of a flexible wiper passing over each nozzle while carrying ink drawn from other nozzles.
Typically, the ink chemistry for color ink is different from that of black inks. Color inks require controlled interaction to provide color mixing or blending, and edge acuity is only moderately important. Black inks, on the other hand, require good edge acuity for printing detail-critical images and text. In addition, for printing a combination of color and black inks, such as on a page including black text and color photographs, the chemical interaction between the different ink types is important.
For critical applications, black inks are selected to be chemically reactive with the color inks, so that a precipitation reaction occurs when black and color ink droplets meet on a media sheet. Instead of blurring, blending, and compromising edge acuity, the black pigment precipitates out of solution at the location where it was initially directed. Typical reactive ink sets have inks of different pH values, where a black ink may be acidic while the color inks are basic. However, the use of reactive inks will lead to a reaction wherever they mix. Accordingly, printers using such inks employ separate pens for each ink type, each with its own print head. This allows wiping of the print heads without mixing of different ink types that would react to generate an increasing quantity of precipitate sludge, which may resist wiping, and which may cause nozzle clogging and other performance and reliability problems.
In less critical applications, low cost printers use non reactive black and color inks in a single pen, where the color and black ink chemistry is the same, or adequately compatible to prevent precipitation during wiping. The use of a single pen eliminates the need for alignment procedures between pens, reduces costs by eliminating many electrical interface connections and mechanical pen-retaining latches, and reduces the number of electrical connections. However, the use of non reactive ink compromises the edge acuity where black ink interacts with color ink, so there is a trade off between cost and performance.
The present invention overcomes the limitations of the prior art by providing an ink jet print cartridge having a first ink chamber containing a first type of ink and a second ink chamber containing a second type of ink. The body has a third fluid chamber containing an unpigmented fluid different from the inks. A printhead attached to the body has a set of orifices associated with each chamber. The inks may be reactive inks, and the third fluid a buffer to prevent precipitation reaction on the print head surface. The orifices for the buffer may be positioned between the orifices for the different inks to prevent mixing during wiping, except in the presence of the buffer.